Investigation into effectiveness of a downwire insulator and insulated bonding wire as mitigation measures for pole-top fires in woodpole structures
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Date
2015-04-08
Authors
Ntshani, Mikhuva Doctor
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Abstract
Woodpole structures are widely used for overhead medium voltage (MV) distribution lines in South
Africa. The problem is that wood burning, also known as pole-top fires, occurs on such structures,
resulting in undesirable power outages, replacement costs and safety risks to humans and animals.
The feasibility of a modified partially bonded woodpole structure with a small downwire insulator
instead of the 500 mm gap along the wood in reducing the risk of pole-top fires and bird safety was
investigated. Tests were conducted in a natural heavily polluted environment at the Koeberg Insulator
Test Pollution Station (KIPTS), where several structures were erected and energised at 22 kV (phaseto-
phase). Laboratory tests were also performed to confirm the findings obtained at KIPTS. The
downwire insulators have been proven to effectively conduct leakage current to earth under polluted
and wet conditions, with very little current flowing through the woodpole. Furthermore, no tracking
and/or burning was observed on the woodpoles of all the test structures. The findings are promising
and indicate that a downwire insulator may be effective in reducing the risk of pole-top fires. The
downfall is that the impedance of the downwire insulators and insulation levels provided by the
downwire insulators under such conditions may not be adequate to ensure acceptably low risk of bird
electrocution. It may be feasible to design such an insulator that only conducts leakage current when
necessary to prevent pole-top fires, while maintaining sufficiently high impedance to prevent bird
electrocution. The preliminary specifications of the new downwire are provided.